Solvent recovery from smokeless powder



April 28,` 1942. G. E. BESL-:TTI ETAL 2,280,803

SOLVENT RECOVERYl FROM SMOKELESS POWDER Filed Feb. 2s, .1939

WaiierAeW 6270' @5927i INVENToRs l'ZZ MaoKeyBY .ATTORNEY Patented lApr. 28, 1942 UNITED STATES PATENT O SOLVENT RECOVERY FROM SMOKELESS FFICE POWDR Gild Edward Desetti, Woodbury, N. J., and Walter Arthur Dew and Bill Harry Mackey, Wilmington, Del., assignors to E.`I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application February 23, 1939, Serial No. 257,840

Claims.

without causing deterioration in the physical appearance of the dried material. Such diiliculty has been particularly important in the smokeless powder art, in view of the fact that the ballistic performances of the various smoke less powders are influenced greatly by the physical dimensions of the grains of powder. Moreover, non-uniformity in the solvent content of the various grains of powder, after the solvent recovery treatment, is considered disadvantageous because it may necessitate additional precaution in the subsequent processing of the material. Thus, to obtain the uniform performance characteristic of a satisfactory powder, care is exercised throughout the steps involved in its manufacture, in order that the iinished grains possess definite dimensions. The removal of solvent, as one of the steps in the manufacture of smokeless powder, likewise has been iniluenced by the prerequisite of substantially exact dimensions of the powder grains, and conl sequently the many modifications eiected in the removal of solvent have always been made subservient to said prerequisite.

Moreover, a green powder grain is dillcult to dry with the subsequent production of a grain free from distortion. By the term green powder, we intend to .designate powder recently formed into grains and having a high solvent content, namely, a freshly granulated powder which has not been subjected to any prior drying operation, and this term is used in this sense throughout the descrption of the invention. This difficulty in production of a suitable grain is probably due to the fact that the ends of the usual powder grain tend 'to dry more rapidly Another object is a process of removing solvent in lli such manner that a powder of substantially uniform properties results. A further object is a safe and emcient process for recovering solvent from smokeless powder. Other objects will become apparent as the invention is described hereinafter.

We have found that the foregoing objects are accomplished by circulating` a drying medium past, and in contact with, the green smokeless powder from which the solvent is to be removed Aand controlling the temperature of the drying medium during its flow through the bed of smokeless powder, so that the difference between the temperature of said drying medium entering the powder bed, and that exiting therefrom, does not exceed a predetermined value. This limitation in temperature differential is not absolutely fixed, but rather depends somewhat on the particular type of power treated, both the physical properties and the composition of the powder being influencing factors. In the most preferred form of our invention, we contemplate characterized by a smooth inner surface, that is,

one free from crevices and the like, so that the possibility of accumulation of inflammable ingredients is reduced to a minimum. Elective circulation of the drying medium is attained by means of the blower 2 through the various pipes. in theV direction indicated. Connected to the powder container are the cooler 3 and the heater 4. The amount of gas passing through the cooler -is controlled by means of the dempers 5, whereas the solvent condensed from the sas passing therethrough exits by means of outlet 0. The various units, as well as the interconnecting pipes. are insulated wherever necessary.

The general scheme of operation is as follows. The solvent-laden powder is charged into the powder container, and is `heated very gradually by circulation of the drying medium through the heat exchanger and continuously passing the drying` medium through the powder to be dried, until the powder has attained the predetermined maximum temperature of operation. During this gradual elevation of temperature aportion of the solvent-containing gas may be passed through the cooler, with the result that some of the solvent is condensed and separated therefrom. Once thepowder has reached the desired temperature, this temperature is maintained by suitable .regulation of the heat input via the heater. This procedure is usually continued until the solvent content oi the powder has been decreased to a predetermined value. At any time during the operation, the temperatures of the heater and cooler may be varied between rather wide limits, if such procedure is an aid in maintaining the optimum conditions for the particular type of powder being treated.v

,Various factors may be employed to effect control of the temperature gradient occurring during the passage of the drying medium through the bed of solvent-laden powder. Among these factors may be mentioned the regulation of the temperature of the incoming drying medium with respect to the temperature of the powder itself; the rate, of circulation of the drying medium which is important because of the time available for cooling said medium; and the solvent vapor content of the incoming drying medium.

The preferred embodiment of the process contemplates regulation of the solvent vapor content of the incoming drying medium by passing only a predetermined portion of said medium through the condenser, which portion, however, may vary during the operation, together with a high flow of the fluid medium. This combination makes possible the maintenance of substantially uniform temperatures and rates of evaporation from grains of powder at all loci Within the system. It will be understood that the requisite rate of flow Will be inuenced by several factors, such as the depth of the powder bed, for example. The following examples are illustrative and are not' intended as limiting the invention.

Eample 1 lof ether and alcohol, of approximately 401%.

The gaseous drying medium was circulated through said bed of powder at approximately room temperature. During the rst eight hours there was no change in temperature, the'gas `within the system merely being circulated by means of the blower, the rate of ow of said Vgas through the powder container being about 0.5 cu. ft. per minute per pound of dry powder. The dampers in the system were xed so that only 8% of the gas leaving the powder chamber passed through the cooler, the remaining portion of the gas being passed directly from the chamber through the blower to the heater. 'I'his d: vision of the-drying medium was maintained throughout the operation. l

At the end of the 8-hour period, the temperature of the powder was raised 2 C. per hour until it reached 55 C., this temperature being maintained constant during the remainder of the operation. The total time of treatment was '72 hours. Temperature measurements near the top and bottom of the powder bed were made at frequent intervals during the entire operation. The maximum diierences in temperatures between those toward the top of the powder bed, which were constantly higher, and those near the bottom, at diierent stages of the operation, were as follows:

Analyses of numerous samples of the powder treated as above revealed that the average solvent remaining therein was about 5.0%.

Example` 2 The sample treated in this experiment was a. green smokeless powder containing nitrocellulose of 12.60% nitrogen content, having a granulation suitable for a 5-inch gun (mean web .045 inch). The content of solvent, alcohol, and ether at the time of charging was about 45% by weight. As charged, the bed of powder formed had a depth of 38 inches. The mode of treatment was identical with that described in the foregoing example, except that the rate of ilow of the drying medium through the powder was 0.55 cu. ft. per minute per pound dry powder. The solvent content of the powder, after treatment, averaged 8.2%. Temperature measurements toward the top and base of the powder bed showed the following maximum diierences during diierent stages of the operation.

Stage of 'operation Mmium Initial 3.9 Intermediate l0. 0 Toward end oi treatment 2.1

Example 3 The green powder employed in this instance contained nitrocellulose having a nitrogen content of 13.15%, and was granulated for use in a 12-inch gun (mean web .110 inch). The volatile solvent content of this powder was 36.0% by weight. When loaded into the container, a layer of powder having a depth of 36 inches resulted. During the rst 24 hours, there was no change in temperature, the temperature of the powder being held constant. Then the temperature was raised 0.5 C. per hour until 45 C. was attained, which temperature was held Constant for 30 hours, following which the temperature ofl the powder was increased 1.0. C. per hour until 55 -C. was reached, and this temperature maintained during the remainder of the operation. 'I'he overall time of treatment for the removal of solvent. was 144 hours. Solvent content of the powder so treated averaged 7.2%.

At the start of the run, 8% of the -gas circulated Was passed through the condenser, and this proportion was maintained except for the last 24 hours, during which time only 4% of the gas was passed through the condenser. Rate of ow of the gas through the powder chamber was approximately 0.55 cu. it. per minute per pound dry powder.

Stage of operation alum In each of the foregoing examples, the resulting powder was very satisfactory in appearance.

Moreover, in each case, analyses made on various been substantially uniform, because the variations in the solvent remaining in the various powder samples were within the limits of experimental error. ertes are important factors in the procurement of a satisfactory finished smokeless powder.

'I'he process described herein permits removal oi.' solvent from green smokeless powder at a'rate which is quite satisfactory, without yielding a powder of non-uniform physical characteristics. In addition, the removal of solvent from the numerous grains of powder treated is substantially uniform, which is a distinct advantage with respect to the subsequent treatments to be accorded the powder.

It will be obvious to those skilled in the art that variations may be made in our process, without departing from the spirit or scope thereof.

As an example, said process is not limited to the temperatures disclosed in the speciiic embodiments, but rather contemplates the adoption of any temperature deemed safe by those skilled in the smokeless powder art. Likewise, the drying medium may comprise'any gas or vapor, or mixture thereof, which has no deleterious effect upon the material undergoing treatment. We intend, therefore, to be limited only in accordance with the following claims.

We claim:

1. A method of'recovering solvent from freshly granulated smokeless powder which comprises forming a substantial bed of said freshly granulated powder having a depth oi the order of magnitude of at least approximately y36", passing through said bed oi' powder a drying medium having a temperature exceeding that of said powder at a rate oi.' at least @do cubic feet per minute per pound of dry powder, while controlling the solvent carrying capacity of said medium so that the decrease in temperature oi' the drying medium during its passage through said bed of powder is never more than 15 C.

As stated heretofore, these prop- 2. A method of recovering solvent from freshly granulated smokeless powder containing at least `36% by weight of volatile solvent, which comprises forming a substantial bed of such powder, and passing a drying medium having a temperature exceeding that of said powder through said bed at a rate of at least L710 cubic feet per minute per pound of dry powder while controlling the solvent carrying capacity of said medium so that the decrease in temperature -of the drying medium during its passage through said powder is never more than 15 C.

3. A method of recovering solvent from freshly granulated smokeless powder, which comprises forming a substantial bed of said powder having a volatile solvent content of from 36 to 45%` by weight, passing a drying medium having a temperature exceeding that of the freshly granulated powder through said bedat a rate of at least An cubic l'eet per minute per pound of dry powder while controlling solvent carrying capacity of said medium so that the decrease inv temperature of the drying medium during its passage through said bed of powder is never more than 15 C.

4. A method of recovering solvent from green smokeless powder, which comprises formingl a bed of smokeless powder having a volatile solvent content of from 36 to 45% by weight, passing a hot drying medium having a temperature exceeding that of said powder through said bed of powder at a rate of at least 1360 cubic feet per minute per pound of dry powder While controlling the solvent carrying capacity oi' said drying medium so that the decrease in temperature of the drying medium during its passage through said bed of powder is never more than 15 C., and continuing said control until the volatile solvent content of the powder has been reduced to approximately 5% by weight.

5. A method of recovering solvent from green -smokeless powder, which comprises forming a bed of pressed and out powder directly after it has been granulated and havinga volatile solvent content of from 36 to 45% by weight, passing a hot drying medium having a temperature exceedingv that of said powder through said bed of powder at a rate of at least 3io cubic feet per minute per pound o1' dry powder while controlling the solvent carrying capacity of said medium 60-so that the decrease in temperature of the drying medium during its passage through said bed of powder is never more than'15 C., and continuing said control until the. volatile solvent content of the powder has been reduced to approximately 5% by weight. Y

. GILD EDWARD DESE'I'I'I.

WALTER ARTHUR DEW.

BILL HARRY MACKEY. 

